EPTT 2020

12th Spring School on Transition and Turbulence

INVESTIGATION OF TRANSIENT REGIME IN LAMINAR SEPARATION BUBBLE FORMATION

Submission Author: Pedro Bruno Pereira Panisset , RJ , Brazil
Co-Authors: Pedro Bruno Pereira Panisset, omar elias horna pinedo, Igor Braga de Paula
Presenter: Pedro Bruno Pereira Panisset

doi://10.26678/ABCM.EPTT2020.EPT20-0123

 

Abstract

The transient flow regime related with the formation of laminar separation bubbles (LSB) is examined by time resolved measurements of the velocity field. The investigated scenario aims at studying the flow in multiple stage turbines, where the wake of airfoils in previous compression stages induce periodic variation in turbulence level of subsequent airfoil. This might lead to periodic removal and formation of LSB. Here this process is reproduced by excitation of controlled disturbances using a vibrating ribbon. Experiments are carried out in a laminar water channel at PUC-Rio. Measurements are performed on a flat plate subjected to an adverse pressure gradient. The pressure gradient is set by false walls with adjustable geometry. Suction is applied on the false wall, in order to avoid the boundary layer separation at this surface. Bubble topology and disturbance growth along the streamwise direction are measured during the transient of bubble formation. Results show spatial amplification over a narrow frequency bandwidth. Dominant non dimensional frequencies (St=fδ*s/U) at late stages of bubble formation are in close agreement with those reported in literature. During the transient the intensity of reverse flow is significantly changed, hence modifying the stability of base flow.

Keywords

Laminar separation bubble, Boundary Layer Transition, hydrodynamic stability

 

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